Steel for pipeline

ABSTRACT

The present invention provides a steel for pipeline which has excellent strength and toughness at room temperature and high temperature without the necessity of adding expensive alloy elements. The steel for pipeline of present invention comprises 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, simultaneously satisfying expression:Al(N−Ti/3.4)≦0.00015, and is constituted of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof. The steel for pipe of the present invention is suitably for the use as the flow line in severe environment such as seabed oil fields or the like, and can be produced at low cost.

TECHNICAL FIELD

The present invention relates to steel for pipeline which is highly tensile steel having excellent strength, toughness and, particularly, high-temperature strength.

TECHNICAL BACKGROUND

Steel used for pipeline is required to have high strength and excellent toughness as well. Particularly, since the exploitation of oil field and natural gas field has been transferring to more severe environment, the steel is even required to have excellent strength under high temperatures around 200° C.

In consideration of the anticipated exhaustion of the resources of crude oil and is natural gas in the near future, the exploitation of oil fields and natural gas fields have been transferring to more severe environment such as deep-seated oil field, and the product fluids are under the state of high temperature and high pressure.

On the other hand, although the exploitation of seabed oil field is becoming popular, the comparatively small oil fields do not hold their own product fluid treating equipments from the viewpoint of reducing equipments investment and maintenance expense. Therefore, a currently main treating mode is to transport product fluids through seabed pipelines so-called “flow line” to an oil field nearby and carry out convergence treatments. In this case, since high-pressure and high-temperature (until about 200° C.) fluids flow in the flow lines, the pipelines used therefor are required to have excellent strength under high temperature until about 200° C. As a measure to solve this problem, Japanese Patent Laid-Open No. S56-166324 has published a method wherein a seamless steel pipe, which comprises either or both of Mo and V added to a special composition containing C, Si, Mn and Al, is made by hot rolling, and then immediately subject to direct quenching, and tempering thereafter. However, according to this method, the addition of expensive alloy elements Mo and V is indispensable. Further, austenite crystal grains would become large due to the direct quenching and thus impairing the toughness of base metal. Therefore, the pipes made by this method are not suitable to be used in cold districts.

In addition, Japanese Patent Laid-Open No. H02-50917 has published a method of using Mannesman pipe-making process to form a seamless steel pipe from a round ingot having a specific composition with Cr and V as its necessary added elements, and subjecting the steel pipe to heating at 850 to 1000° C., quenching, and then tempering at temperature from 500 to 700° C. The steel pipe made according to this method is still expensive since the addition of expensive alloy elements Cr and V is indispensable.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide steel for pipeline which has excellent strength and toughness at both high-temperature and room temperature without the addition of expensive alloy elements.

In order to solve the problems described above, the inventor made the present invention on the basis of the following findings concluded from repeated researches:

(1) In order to obtain steel used for pipeline with excellent strength and toughness at room temperature and high temperature without addition of expensive alloy elements, the structure of the steel can be made to comprise martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

(2) Although, generally, the structure having such large austenite average grain sizes would impair the toughness of the steel, the excellent toughness can still be obtained without making fine-grain structure but by controlling the addition amounts of Al, Ti and N.

The present invention provides the steel for pipeline as follows.

(1) Steel for pipeline comprising, by mass percent, 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, and simultaneously satisfying expression (1): Al(N−Ti/3.4)≦0.00015  (1) and said steel for pipeline being constituting of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

(2) The steel for pipeline according to (1) further comprising, in addition to the components described above, one or more of the components, by mass percent, not greater than 0.6% of Cr, not greater than 0.3% of Mo, not greater than 0.4% of Cu, not greater than 0.4% of Ni, not greater than 0.01% of Nb, not greater than 0.08% of V and not greater than 0.001% of B.

(3) The steel for pipeline according to (1) or (2) further comprising, by mass percent, not greater than 0.005% of Ca.

BEST MODES OF THE INVENTION

The best modes of the highly tensile steel for pipeline according to the present invention are described in details as follows.

First, the reasons for limiting the chemical composition of the steel in the present invention are described. In this specification, “%” generally represents “mass %” unless there is other specific explanation.

C is an indispensably added element for improving the strength of steel. In order to ensure the required strength of steel at room temperature and high temperature, the addition of C should be 0.04% or more. However, if the C content exceeds 0.16%, the toughness of base metal and heat affected zones at welded joints would be impaired. Therefore, the C content is defined within a range of 0.04 to 0.16%, and preferably 0.06 to 0.13%.

Si is added for the purpose of deoxidization, and it also contributes to the improvement of strength at room temperature and high temperature. However, if the Si content exceeds 0.5%, the toughness of base metal and heat affected zones at welded joints would be impaired. Therefore, the Si content is defined not greater than 0.5%, and preferably not greater than 0.35%.

Mn is an effective element for ensuring the strength and toughness. If the Mn content is lower than 0.8%, it is unable to realize desirable effects. On the other hand, if the Mn content exceeds 1.7%, the toughness of base metal would decrease. Therefore, the Mn content is defined within a range of 0.8 to 1.7%.

P exists in steel as an impurity that impairs the toughness of base metal. Therefore, the P content is desirably controlled as low as possible, not greater than 0.015%, and preferably not greater than 0.012%.

S, similar to P, exists in steel as an impurity that impairs the toughness of base metal. Therefore, the S content is desirably controlled as low as possible, not greater than 0.003%, and preferably not greater than 0.001%.

Ti, in general, combines with N and C contained in the steel and thus existing in forms of TiN and TiC. Either TiN or TiC would impair the toughness when the structure of base metal is coarse. So, from this aspect of view, Ti is preferably not added. But, on the other hand, those N not fixed in the form of TiN would combine with Al to perform AlN which would impair the toughness of base metal. Further, the excessive existence of TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decreases the hardness of steel, thus failing to obtain sufficient strength. Therefore, first, the addition of Ti should be controlled to satisfy expression (1). Secondly, even though the Ti content is controlled to satisfy expression (1), the toughness would still be decreased when it exceeds 0.04%, so that the upper limit of Ti content is defined to 0.04%.

Al is added for the purpose of deoxidization. The Al content lower than 0.001% would cause insufficient deoxidization, and thus bringing about steel deterioration. However, when the Al content exceeds 0.07%, the deoxidization effect does not change any longer, while the content of impurities is increased, and thus decreasing the toughness of steel. Therefore, the Al content is defined within a range of 0.001 to 0.07%, and preferably 0.01 to 0.05%. However, even if the Al content falls within this range, the pinning effect caused by excess of AlN formed by the combination of Al and N in steel would induce the refinement of austenite grains, and thus decreasing the hardness of steel. Thereby, sufficient strength of steel cannot be obtained. Further, excessive AlN in such coarse grain structure would degrade the toughness of base metal. Therefore, the Al content should be controlled to satisfy expression (1).

N exists in steel as an impurity that combines with Ti to form TiN, and those not fixed in the form of TiN combines with Al to form AlN. The existence of AlN and TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decreases the hardness of steel, thus failing to obtain sufficient strength. Meanwhile, TiN and AlN in the coarse structure would degrade the toughness of base metal. Therefore, the N content is desirably as low as possible, not greater than 0.01% and simultaneously satisfying expression (1).

Next, the reasons for limiting the contents of Al, N and Ti according to expression (1) are described.

As described above, Al and Ti combine with N contained in steel to form AlN and TiN. Either of AlN and TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decrease the hardness of steel, thus failing to obtain sufficient strength. Meanwhile, TiN and AlN in the coarse structure would degrade the toughness of base metal. Therefore, in addition to the individual limitation of the contents of these elements, the value obtained from expression (1) should be controlled at not greater than 0.00015, so as to ensure the high strength and high toughness of the steel.

Next, the reasons for adding Cr, Mo, Cu, Ni, Nb, V, B and Ca as desirably added elements are described as follows. Any one of these elements is added for improving the strength of the steel. The strength and toughness of steel are expected to be further improved by addition of one or more of these elements to the basic essential components of the steel.

Cr is an element contributing to high strength by improving hardness. The addition of Cr contributes to high strength at room temperature and high temperature. However, when the Cr content exceeds 0.6%, it would impair the toughness of heated affected zones at welded joints. Therefore, the Cr content is controlled at not greater than 0.6%.

Mo, similar to Cr, is an element contributing to high strength by improving hardness. The addition of Mo can contribute to high strength at room temperature and high temperature. However, when the Mo content exceeds 0.3%, it would impair the toughness of heated affected zones at welded joints. Therefore, the Mo content is controlled at not greater than 0.3%.

Cu can improve corrosion resistance and high strength. However, its excessive addition would impair the field weldability and increase material cost. Therefore, the Cu content is controlled at not greater than 0.4%.

Ni contributes to the improvement of strength without impairing the toughness. However, its excessive addition would impair the field weldability and increase material cost. Therefore, the Ni content is controlled at not greater than 0.4%.

Nb can improve strength by promoting the precipitation. However, when its addition exceeds 0.01%, it would impair the toughness. Therefore, the Nb content is controlled at not greater than 0.01%.

V can improve strength at room temperature and high temperature by promoting the precipitation. However, when its addition exceeds 0.08%, it would impair the toughness. Therefore, the V content is controlled at not greater than 0.08%.

Slight addition of B can ameliorate strength by improving hardness. However, when its addition exceeds 0.001%, it would impair the toughness of base metal and heat affected zones at welded joints. Therefore, the B content is controlled at not greater than 0.001%.

Ca reacting with S contained in steel to form sulfide but it maintains the spherical shape after rolling without extending along the rolling direction. Therefore, the generation of hydrogen-inducing cracking or the like that starts from tips of extended impurities such as MnS can be suppressed. However, the excessive addition of Ca would deteriorate affect the cleanness of steel and impair the toughness of base metal. Therefore, the Ca content is controlled at not greater than 0.005%.

Next, the reasons for limiting the structure of the steel are described.

By making the structure steel for pipeline comprise martensite or bainite with an austenite average grain size of 30 μm or larger, or their tempered structure, excellent strength at room temperature and high temperature can be obtained without adding alloy elements such as Cr, Mo and V. However, when austenite average grain size is greater than 200 μm, the toughness would become lower even if the chemical composition is defined. Therefore, the structure of steel of the present invention is defined to be the structure comprising martensite or bainite having an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

There is no any special limitation on the manufacturing method of the steel for pipeline according to the present invention provided that the steel has the composition and the structure prescribed in the present invention.

The effects of the present invention are further described using the examples shown as follows, but these examples would not introduce any limitation to the present invention.

EXAMPLES

Each of the steel having the chemical compositions shown in Table 1 was melt in a vacuum-melt furnace, and the molten steel is solidified to produce a round ingot with a weight of 50 Kg. The ingot thus obtained was subjected to hot rolling and heat treatment to produce steel plate under the condition shown in Table 2.

Then, JIS Z2202 V-notch Charpy impact test piece and JIS Z2201 round stick tensile test piece were taken from each of said steel plates along the plumb direction of rolling, and then subjected to the Charpy impact test and Tensile test at room temperature and high temperature. The tensile test at high temperature was carried out according to the standard of JIS G0567 with round stick tensile test pieces having a parallel portion space of 6 mm and a punctuation space of 30 mm.

The test results are shown in Table 2. Examples marked with 1 to 17 in Table 2 showed tensile strength greater than 500 MPa at room temperature and high temperatures till 200° C., and fracture transition temperature of −70° C. and lower.

Comparative examples marked with 18 to 29 in table 2, which had components exceeding the content ranges of the present invention, showed lower strength and toughness.

Comparative examples marked with 30 and 31 in table 2, which had the components within the content ranges of the present invention while the austenite average grain size exceeding the range of the present invention, also showed lower strength and toughness.

Comparative examples marked with 32 in table 2, which had the components within the content ranges of the present invention while being subjected normalizing as the heating treatment after rolling, also showed lower strength since the structure comprising martensite or bainite was not obtained.

EFFECTS OF THE INVENTION

The steel for pipeline according to the present invention has excellent strength and toughness at room temperature and high temperature without the necessity of adding expensive alloy elements, and therefore realizing high production efficiency and low production cost. TABLE 1 Components (mass %) Steel Expression Sort Mark C Si Mn P S Ti Al N (1) Present A 0.07 0.27 1.38 0.011 0.001 0.006 0.036 0.0039 0.00008 Invention B 0.16 0.25 1.24 0.013 0.001 0.007 0.038 0.0037 0.00006 C 0.08 0.46 1.35 0.015 0.001 0.008 0.041 0.0040 0.00007 D 0.11 0.30 0.83 0.010 0.001 0.009 0.033 0.0038 0.00004 E 0.07 0.28 1.64 0.010 0.001 0.008 0.033 0.0038 0.00005 F 0.07 0.30 1.43 0.010 0.003 0.008 0.033 0.0038 0.00005 G 0.07 0.27 1.37 0.011 0.002 0.034 0.062 0.0081 −0.00012 H 0.06 0.25 1.25 0.012 0.001 0.007 0.033 0.0036 0.00005 I 0.05 0.25 1.31 0.012 0.001 0.010 0.033 0.0036 0.00002 J 0.06 0.26 1.24 0.013 0.001 0.006 0.036 0.0035 0.00006 K 0.07 0.26 1.25 0.015 0.001 0.008 0.038 0.0036 0.00005 L 0.06 0.27 1.26 0.011 0.002 0.008 0.035 0.0033 0.00003 M 0.12 0.26 1.32 0.014 0.001 0.008 0.034 0.0040 0.00006 N 0.06 0.27 1.36 0.015 0.001 0.006 0.039 0.0052 0.00013 Comparative O 0.02 0.25 1.30 0.010 0.001 0.006 0.033 0.0038 0.00007 Example P 0.23 0.27 1.32 0.012 0.001 0.007 0.037 0.0046 0.00009 Q 0.07 0.67 1.29 0.013 0.001 0.006 0.035 0.0036 0.00006 R 0.07 0.25 0.71 0.011 0.001 0.006 0.036 0.0031 0.00005 S 0.06 0.27 1.85 0.011 0.001 0.007 0.037 0.0037 0.00006 T 0.06 0.21 1.35 0.021 0.001 0.005 0.040 0.0028 0.00005 U 0.08 0.23 1.31 0.013 0.006 0.006 0.038 0.0037 0.00007 V 0.05 0.24 1.32 0.011 0.001 0.051 0.041 0.0039 −0.00046 W 0.06 0.22 1.28 0.010 0.002 0.007 0.0004 0.0042 0.00000 X 0.07 0.25 1.33 0.013 0.001 0.008 0.082 0.0054 0.00025 Y 0.06 0.24 1.30 0.012 0.001 0.007 0.037 0.0118 0.00036 Z 0.05 0.22 1.26 0.012 0.001 0.005 0.053 0.0063 0.00026 Components (mass %) Steel Others Sort Mark Cr Mo Cu Ni Nb V B Ca Present A — — — — — — — — Invention B — — — — — — — — C — — — — — — — — D — — — — — — — — E — — — — — — — — F — — — — — — — — G — — — — — — — — H 0.52 — — — — — — — I — 0.25 — — — — — — J 0.16 — — — — 0.06 — — K — — 0.29 0.25 — — — — L — — — — 0.007 — — — M — — — — — — 0.0007 N — — — — — — — 0.0035 Comparative O — — — — — — — — Example P — — — — — — — — Q — — — — — — — — R — — — — — — — — S — — — — — — — — T — — — — — — — — U — — — — — — — — V — — — — — — — — W — — — — — — — — X — — — — — — — — Y — — — — — — — — Z — — — — — — — —

TABLE 2 Wall thickness Finish Of Average Strength at Strength at Temp Steel slab Heat Size of room temp. high temp. Fracture Test Heat after after treatment Tempering Austentite Yield Tensile Yield Tensile Transition Piece Steel Temp rolling Rolling after Temp. Grains Strength Strength Strength Strength Temp. Sort Mark Mark ° C. ° C. ° C. rolling ° C. μm MPa MPa MPa MPa ° C. Present 1 A 1250 1050 25.4 Quench in water — 66 591 643 573 612 −72 Invention 2 A 1250 1050 25.4 Quench in water 650 70 473 559 451 534 −86 3 A 1250 1150 25.4 Quench in water 650 92 520 604 497 577 −78 4 A 1250 950 25.4 Quench in water 650 54 449 537 429 513 −91 5 B 1250 1050 25.4 Quench in water 650 72 519 629 485 591 −70 6 C 1250 1050 25.4 Quench in water 650 66 475 558 449 529 −84 7 D 1250 1050 25.4 Quench in water 650 62 407 528 391 504 −90 8 E 1250 1050 25.4 Quench in water 650 67 529 594 508 567 −74 9 F 1250 1050 25.4 Quench in water 650 68 476 547 456 522 −82 10 G 1250 1050 25.4 Quench in water 650 65 511 574 491 549 −80 11 H 1250 1050 25.4 Quench in water 650 69 513 570 491 544 −70 12 I 1250 1050 25.4 Quench in water 650 71 494 564 465 531 −76 13 J 1250 1050 25.4 Quench in water 650 61 543 609 515 584 −75 14 K 1250 1050 25.4 Quench in water 650 66 500 582 478 553 −83 15 L 1250 1050 25.4 Quench in water 650 47 490 567 466 536 −73 16 M 1250 1050 25.4 Quench in water 650 68 503 603 475 573 −71 17 N 1250 1050 25.4 Quench in water 650 39 444 526 422 505 −101 Comparative 18 O 1250 1050 25.4 Quench in water 650 62 391 447 366 418 −112 Example 19 P 1250 1050 25.4 Quench in water 650 59 628 701 587 659 −41 20 Q 1250 1050 25.4 Quench in water 650 66 468 550 437 513 −59 21 R 1250 1050 25.4 Quench in water 650 65 346 455 321 423 −98 22 S 1250 1050 25.4 Quench in water 650 67 573 637 539 600 −49 23 T 1250 1050 25.4 Quench in water 650 67 471 551 438 513 −52 24 U 1250 1050 25.4 Quench in water 650 63 464 555 431 518 −51 25 V 1250 1050 25.4 Quench in water 650 35 719 757 674 709 −39 26 W 1250 1050 25.4 Quench in water 650 77 456 546 426 511 −45 27 X 1250 800 25.4 Quench in water 650 21 327 413 314 396 −35 28 Y 1250 1200 25.4 Quench in water 650 17 249 364 239 350 −48 29 Z 1250 1050 25.4 Quench in water 650 20 305 397 293 381 −44 30 A 1250 800 25.4 Quench in water 650 12 345 419 330 398 −90 31 A 1250 1200 25.4 Quench in water 650 263 756 839 709 790 5 32 A 1250 1050 25.4 normal 650 67 260 336 239 305 −94 

1. Steel for pipeline comprising, by mass percent, 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, and simultaneously satisfying expression (1): Al(N−Ti/3.4)≦0.00015  (1) and said steel for pipeline being constituted of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.
 2. The steel for pipeline according to claim 1, further comprising one or more, by mass percent, not greater than 0.6% of Cr, not greater than 0.3% of Mo, not greater than 0.4% of Cu, not greater than 0.4% of Ni, not greater than 0.01% of Nb, not greater than 0.08% of V and not greater than 0.001% of B.
 3. The steel for pipeline according to claim 1, further comprising, by mass percent, not greater than 0.005% of Ca.
 4. The steel for pipeline according to claim 2, further comprising, by mass percent, not greater than 0.005% of Ca. 